1. Field of the Invention
The present invention relates to an image forming apparatus capable of stably controlling image density regardless of changes in various conditions.
2. Description of the Background Art
In an image forming apparatus, such as a copier, a laser beam printer, etc., using an electro photographic system, image density is stabilized at all times as described in Japanese Patent Application Laid Open No. 2004-354623 (JP-2004-354623-A). Specifically, image density is controlled such that a gradation pattern is initially formed including multiple toner patches on an image bearer, such as a photoconductor, etc., under different image forming conditions (e.g. developing potentials) so that an amount of toner particles adhered to the respective patches becomes different from each other. Subsequently, an optical sensor serving as an adhered toner amount detector detects each of the toner patches, and the amounts of toner adhered thereto are calculated based on detected values using a predetermined adhered amount calculating algorithm. Subsequently, a linear equation (y=ax+b) is obtained based on a relation between the amounts of adhered toner particles and the image forming conditions of the toner patches. Then, development γ serving as an index value indicating developing performance (the slope of a line plotted against the developing potential is represented by a horizontal axis and the amount of adhered toner particles is represented by a vertical axis) and a development start voltage Vk (the intercept (−b/a) of the horizontal axis) are found. Based on the thus found development γ and development start voltage Vk, image forming conditions, such as LD power, a charging bias, a developing bias, etc., are adjusted so that the developing potential can adhere an adequate amount of toner particles.
In a color image forming apparatus which forms a color image using toner particles of four colors of Y, M, C and K, gradation patterns are formed for respective colors. In such a situation, when only one optical sensor is used, the gradation patterns of these colors are accordingly formed aligned in a sub-scanning direction resulting in slow image density control. To resolve the problem, Japanese Patent Application Laid Open No. 2006-234862 (JP-2006-234862-A) discloses an image forming apparatus that employs a so-called tandem system, in which Y, M, C and K image forming stations each having a photoconductor are arranged in parallel to each other along a traveling direction of an intermediate transfer belt serving as an image bearer. Subsequently, the color image forming apparatus detects the gradation patterns of respective colors with Yellow, Magenta, Cyan, and Black optical sensors aligned in the main scanning direction facing the intermediate transfer belt. Specifically, the gradation patterns of respective colors are formed on the intermediate transfer belt aligned in the main scanning direction to be detected by multiple optical sensors corresponding to an applicable color. Consequently, it is possible to decrease the time for the image density control.
However, density fluctuates in the main scanning direction in the image forming apparatus due to various factors, such as developing gap fluctuation where the developing gap gradually increases from one to the other ends of the photoconductor in its axial direction, etc. Such density fluctuation causes the following problems. When the density fluctuates in the main scanning direction and is darker at a section on the intermediate transfer belt where a gradation pattern is formed with a certain color, than the other sections, the gradation pattern of the certain color increasingly attracts toner particles due to the fluctuation in the density. As a result, when image density is controlled based on the detection result of the above-described gradation pattern, although an image formed under the thus executed image density control achieves a target density at the section forming the gradation pattern, density of the other sections become thinner than the target. Accordingly, an image of the color becomes thinner as a whole after the image density control.
By contrast, when the density fluctuates in the main scanning direction and is darker at the other section on the intermediate transfer belt than the section where a gradation pattern is formed with a certain color, the gradation pattern of the certain color attracts fewer toner particles due to the fluctuation in the density. Therefore, when the image density is controlled based on the detection result of this gradation pattern, although a section of an image where the gradation pattern is formed achieves a target density after the image density control, the other section is darker than the target. As a result, an image of the color becomes darker as a whole after the image density control.
According to Japanese Patent Application Laid Open No. 2008-139592 (JP-2008-139592-A), first and second optical sensors are arranged on both sides of an intermediate transfer belt, and a gradation pattern is formed and detected by differentially. Subsequently, the image forming apparatus averages the previous and current detection results and adjusts image density based on the average value thus obtained. Accordingly, it is possible to prevent the image from becoming thinner or darker as a whole after the image density control. Because, even though both more and less than the average of toner adhering amounts are obtained by both side optical sensors due to fluctuation in the density in the main scanning direction but are averaged.
However, according to the image forming apparatus of JP-2008-139592-A, density sometimes fluctuates in the main scanning direction between previous and current image density control operations, and as a result, an accurate the average is rarely obtained. Accordingly, the image is not sufficiently prevented from becoming thinner or darker as a whole after image density control. In addition, since detection results reflects problems other than the fluctuation in density in the main scanning direction but are averaged at the same time, precision control of image density remains unattained.